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Jupiter

Weather systems on Earth blow themselves out in a few days or weeks. On the solar system's giant planets, though, weather systems can last for decades. The record holder is Jupiter's Great Red Spot, which was first observed no later than 1830 -- and perhaps a good bit earlier. This oval storm system is wide enough to swallow two Earths, and the winds at its perimeter blow several times faster than the strongest hurricane ever recorded on Earth.

As the Great Red Spot demonstrates, Jupiter is a world of superlatives. It is the largest planet in the solar system (big enough to swallow more than 1,300 Earths, and bigger than some types of stars), and more massive than the solar system's other known planets and moons combined. It rotates faster than any other planet, it produces the most powerful magnetic field, and it offers the largest and most interesting assortment of moons.

Despite its great size and its brightness in Earth's night sky, however, much about Jupiter remains poorly understood. Scientists must infer quite a bit about it from observations by the spacecraft that have visited the planet.

Much of the mystery surrounds Jupiter's interior. From the planet's mass, density, and magnetic field, scientists surmise that it probably has a dense core of rock and metal at least 10 times as massive as Earth. A thick layer of hydrogen surrounds the core. Jupiter's gravity squeezes this layer so tightly that it acts as a metal. The metallic hydrogen probably rotates at a different speed from the core, so it produces a "dynamo" effect, generating electric currents that create Jupiter's magnetic field.

Layers of hydrogen and helium gas surround the metallic hydrogen, and a relatively thin layer of clouds envelops the entire planet. (This layer is actually dozens of miles thick, but compared to Jupiter's great size, that's like the skin on an onion.) Jupiter's rapid rotation stretches the clouds into narrow bands that circle the entire planet.

The bands show different colors, which means their clouds are made of different materials and hover at different altitudes. The highest layers of clouds, which are white, are made of frozen ammonia. Clouds in the next layer contain ammonia mixed with other chemicals, so they look brown or orange. The lowest layers contain water vapor and water ice, so they look blue.

These bands form alternating light and dark stripes. The light-colored stripes are decks of clouds that are pushed high into the atmosphere by rising bubbles of warm gas. The darker stripes consist of cooler material that is dropping back into the planet's atmosphere.

Much of the weather in these zones draws its energy not from the Sun, as is the case with storms on Earth, but from deep within Jupiter itself. As Jupiter's gravity squeezes the planet, it produces heat that wells up through the atmosphere then escapes into space as infrared energy. Jupiter radiates more energy into space than it receives from the Sun.

Rings encircle Jupiter, although they are far darker and skimpier than those of the flashier planet Saturn. The rings may consist of material that was "sandblasted" off the surfaces of Jupiter's moons by collisions with meteorites.

Since Jupiter has no solid surface, no human will ever walk on the planet. In fact, any attempt to visit the Jovian system will require heavy shielding. Jupiter's magnetic field captures electrically charged particles from the Sun and from the planet's volcanic moon Io. These particles create strong radiation belts. Around Io's orbit, the radiation belts are powerful enough to kill an unprotected human in a few minutes. That adds one more item to the list of Jupiter's superlatives: deadliest radiation belts.

Viewing Guide

♃The largest planet in the solar system, and usually the brightest object in the night sky after the Moon and Venus, shines at its best in June, when it is brightest and is in the sky all night.

Exploration

Pioneer 10 and 11 blazed the trail to the outer solar system during the 1970s. Pioneer 10 became the first spacecraft to visit any planet beyond the asteroid belt when it flew past Jupiter on December 3, 1973. Pioneer 10 found that Jupiter's interior is hotter than previously thought, discovered that the planet's radiation belts are strong enough to kill a human being, and discovered that Jupiter's moon Io is embedded in a giant cloud of hydrogen that encircles Jupiter. Pioneer 11 flew past Jupiter a year later, then in 1979 became the first craft to encounter Saturn. Since then, several other spacecraft have flown past the giant planet, including Cassini, which used Jupiter's gravity to sling it out to Saturn. The Galileo spacecraft orbited Jupiter and its moons for more than five years.

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The Astro Guides for the Solar System and Beyond the Solar System are supported by the National Aeronautics and Space Administration under Grant Nos. NNG04G131G and NAG5-13147, respectively.